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Jeeper needs Help !!


NAXJA Forum User
Hi there all..

Coming to you from Australia, I need help with my jeep, and the search has gone international.

Have owned 3 xjs over 10 years. Theres alot of jeep hate in Australia, but I love the things.

My current issue has me completely stumped, and unless your a complete jeep enthusiast, i doubt anyone else could pinpoint the answer. You need to know about crank position sensors, that sometimes you need to wiggle the shifter to turn it over, or bash the dash to get the speedo working haha. Xj specific stuff.

Anyway. My issue.

1996 RHD 4.0 lt auto. 200,00KMS.

I have a horrible miss Im chasing. Its pretty damn specific. Ive search the entire web, and these forums, and still cant pin point it. I need xj specialists.

So the miss starts once to operating temp.
When cold, or hot, starts fine.
Idles fine cold or hot.
When started cold, and running normally, i leave my house, boot it to the floor, and it runs as it should. As the symptoms start, I can plant my foot halfway, and there is power. As I go to 3/4 or flat pedal, there is LESS power, like it is bogging down. Ease back to half pedal, and the power comes back and increases. After about 5-10 mins, it starts missing badly, and gets worse quick to the below standard.

Symptoms are:

Foot flat - It completely pauses, before backfiring out the exhaust, stuttering bucking, no power at all, just bucking like a pig.

Foot planted half way:
Surging forward and backwards. No backfiring, but lack of power still.
Consistant surging forward and back.

Light acceleration :
No symptoms. On a flat road cruising along at 90kmh, no miss, nothing.
As soon as you start putting your foot down, you start the surging, and going into the above 2 categories.

Once its got to this stage and I limp it home, Planting the foot in neutral, it will only go to 2000-3000 rpms, at stutter and backfire and wont go any further.
But if I rev it smoothly and a bit slower, it well rev through the entire range.
Plant it : popping, backfiring sputtering.
Slower acceleration : climbs entire rev range.

Just to screw me and a buddy that were trying to diagnose it even more....
Once we got it bucking on the road, got it home, planted foot fast and it was sputtering and backfiring only to 3000rpm....I shut it off for 5 seconds.
Turned it back on. Symptoms gone. Plant foot and screams through the whole rev range. Take it back out on the road, after 5 mins, symptoms return. Come home, plant it....sputtering, backfiring. Turn it off for 5 seconds.....gone.

I'm pulling my hair out on this, and need help desperately.

This is a list of things I have tried so far. I currently own 6 xjs and a zj.
4 of the xjs are parts cars. As much as i worry swapping in older parts, id prefer that than cheap chinese crap. Ive also changed multiple times to completely try and rule them out. If a replaced part was also faulty, i may expect slightly different symptoms.

New Crankshaft sensor x3 ( now have a genuine one in there)
New camshaft sensor x2 (oem)
New leads x3
New sparkplugs x 3 ( have brand new champion copper in now)
New upstream oxygen sensor x 2
New air intake sensor ( cleaned both sensors and mount point)
New Map sensor ( brand new Trident)
New TPS sensor x3
New fuel pump x 3
New fuel filter
New coil x3
New cap and rotor x3
New coolant temperature sender sensor x2
New injectors and rail ( thoroughly cleaned )
New MAF sensor
Drained entire tank and refilled.
Removed throttle body and cleaned entire thing
manifold and throttle body cleaner used
Fuel injector and octane boost run through fuel
No symptoms of heatsoak, I run twin electric thermo fans, have hood scoops and rear of bonnet raised. Its cool as under the hood. Engine temps on the gauge are good, never overheats.
No cracked head symptoms, no oil in water, no water in oil, no overheating, not losing any water, compression seemed good.
We tried to measure fuel pressure with a tyre gauge as its all we had, got about 40-45psi.
Alternator and battery seem got, pumping about 13.7-14.0 volts at idle.
Contact cleaner in all connectors
Redid all ground points.
No apparent vacume leaks.

As you can see, ive gone full parts cannon on this thing. More like parts machine gun.

NOTHING, changed any of the symptoms.

Went and bought an OBD2 reader today and cleared stored codes, as we had been disconnect a heap of stuff. Performed more drives and tests, and got it to misfire. Engine is throwing no codes on obd2 reader or the 3 turn key trick.

Im literally out of ideas at this point.

The reason I am so desperate to get this fixed, is I have my young son coming to stay with me for xmas. I only get to see him 4 times a year because my ex wife moved him 1000kms away :(

But ive got a roof top tent, and he loves camping and 4wding with me, and he has been banging on to me about it on the phone. Ive been at this thing for the last month solid, trying to remedy it before he comes.
I've got 1 week left. I went 10 hours straight on it yesterday, without stopping, without eating.

The miss is still there....

I feel I'm so close. I dont think its a catastrophic failure. I dont think its something to complex. But Im lost.
Anything you guys could suggest would be greatly appreciated. I'm going to paste on a few other forums, as I need all the help I can get with 1 week to go.

So in a nutshell.

Jeep misses, backfires, stutters loses power.
Only does it once warmed up after 10 mins of driving.
A turn off, turn back on restart will fix problem, for another 5-10 mins.
Very light throttle, barely any missing at all, no symptoms.
More the pedal is pressed, worse symptoms get.
Starts and restarts fine
Idles fine.

I apologize that this was a bl@@dy novel, but i really need help, and I know from experience of searching the web, as much information as possible is best.
Personally I think there is a few specific symptoms and signs that differentiate it from all the other threads Ive researched.

But the main reason is so that in a weeks time, I can put a smile on a small boys face, by letting him steer it on the beach, and taking him camping at xmas.

Sincere thanks in advance.
Gently push on the side of the distributor rotor with one finger to see how far the shaft moves.
Check your O2 sensor wires, paying close attention to where the front harness goes behind the steering pump bracket. The rear O2 sensor wires cooking on the exhaust.
Sure sounds like an O2 sensor issue, if it isn't the O2 sensor itself the wiring is suspect.
Thanks for the reply.
Have already checked distributor shaft play, as I seen it in a thread during research.
Shaft has no play.
Have roughly checked over the wires, but I intend on going over them a lot more closely, including the grounds. From what I have read, a malfunctioning, non working or shorted rear 02 sensor shouldnt affect engine running. It will throw a code, but not affect performance. Thats what I read anyway, not sure if thats correct?

Also, mate from today commented tonight that while we were live streaming data from the OBD2 reader today, he was sure he saw vacume drop...

Now Again, i havent gone over the entire vacume system with a fine tooth comb, but nothing jumped out. Tommorow I will connect the scanner and watch the vacume readings, although with all the signs and symptoms I described, I didnt think it lined up with a vacume leak, especially with a restart clearing the issue temporarily. But I could be wrong.
Given all the sensors and other things already taken care of I would be closely inspecting your wiring and your terminals/connections.

One possible explanation is that you are getting higher resistance in a circuit somewhere. The wiring in these vehicles is getting old, and it wasn't designed with much room for error. As the wires and connections get older the resistance increases. You may have gone past the tipping point somewhere in your electrical system. The challenge is to find out where.

Take a look at this thread for an example of what I am talking about: https://www.naxja.org/forum/showthread.php?t=1148570

If you can find any sections of wire that have hardened then you should give consideration to what circuits those relate to and consider how to repair them.

Also pay attention to the cleanliness of the contacts for all plugs. Some aerosol contact cleaner and a small brush may be helpful.

I wish you the best of luck and hope you succeed in taking your son camping. Those times are priceless.
O2 sensors are way slow to throw a code, at least on my 96. They need to be almost completely shot before they throw a code. I surmise a partially melted O2 wire may also be slow to throw a code. Check your O2 heater fuse. It may not be an O2 sensor but the symptoms fit. Cleaning the connectors is a good idea anyway, the one down by the side of the motor is prone to getting oil-soaked and/or grimy. I use one of those toothbrushes designed to go between your teeth and a quality contact spray.
You've got a long list of new sensors but are they genuine Mopar? I personally would be looking very hard at all the wiring and that means separating and inspecting every inch of each wire!
The rear O2 sensor affects fuel trim. its main function is to make sure there is enough leftover oxygen during ignition so the Cat works efficiently. The battery temp sensor in some 96 XJ's also has some effect on fuel trim, how or why I have no idea.
From the sound of things, it sounds like your front O2 sensor is having issues. The lower harness where it goes behind the power steering bracket can be cooking on the exhaust header. It is pretty common and it's hard to see that spot.
Another trouble spot is where the injector harness makes the bend at the firewall. Years of flexing can fatigue the copper core of the wire under the insulation, which is a pain to find. The CPS wire can also flop onto the header if it isn't tied up out of the way. Most CPS have a lot of extra wire.
Another thing that can drive you nuts is a connector pin backing out of a connector. A tiny tab on the side of the connector pin holds it in place, if the tab fails or gets pushed flush the connector pin can back out of the connector. I got in the habit a long time ago of gently pulling on the wires at the rear of the connector every time I have them apart and put them back together again.
Just some ideas, good luck.
You should also put a scanner on it that is capable of live data logging. That would tell you a lot of before/during/after data when it happens.
The symptoms should make one suspect O2 sensors issues, faulty O2 sensors, incorrect generic or Bosch sensors, or sensor wiring issues. The Jeep 4.0L should use NTK O2 sensors.

Faulty O2 sensors, incorrect O2 sensors, and O2 sensor wire harness damage/faults will most often cause bucking, backfiring, and stalling, usually with no trouble codes.

Non-functional or deleted O2 sensors will cause a 30-40% drop in normal gas mileage and the engine will run seemly normally.

Visually inspect and test with volts/ohms multi-meter OBD sensor wiring looking for:
  • Chafed or melted wire insulation especially near sharp metal edges like the valve cover and the firewall, near the exhaust, and near the driveshafts.
  • Damaged wires.
  • Failed wire harness splices, especially on top of the intake manifold.
  • Corroded and bent or pushed back wire plug pins
The no codes is where things get weird for an obd2 rig. Almost gotta be a chafed wire somewhere causing resistance in the computer. Although the vacuum in the tank is an easy one to check.
The no codes is where things get weird for an obd2 rig. Almost gotta be a chafed wire somewhere causing resistance in the computer. Although the vacuum in the tank is an easy one to check.

Yeah, this one happened to me at one point. Upstream O2 wire got partially wrapped in the drive shaft and chaffed. Symptoms were intermittent loss of power and general unsteadiness. No codes.
So went for a drive just now, and while watching data, both sensors are losing power under medium throttle, while still in closed loop.
They will be reading fine, updating readings within correct parameters, a bit more throttle, and they both drop to 0 Voltage, while still in closed loop.
Back off a bit, and the voltage comes back, and the car drives fine again.
WOT results in briefly going to open loop, but symptoms are the same, bucking, backfiring, no power.
So checked vacuum today, it was good. Tank breather line to charcoal canister felt a bit blocked, blew it out. Also changed charcoal cannister for good measure. Downstream sensor was reading 1.0V and not moving. Replaced with a spare NTK sensor, and got it reading. NO change.
Checked all wiring to both sensors, both plugs. I couldnt see any faulty wires, no splits, cuts etc.
Put brand new conduit over both.
Undid the ground by the dipstick, cleaned, reattached.
No change.
Am I right that in a closed loop, sensors should always read voltage, and not drop to zero?
So annoying on first start up, it runs perfect in open loop, and perfect in closed loop.
But after 10 mins, it goes to crap. Then I can turn it on and off, and its fine again.

Whatever it is, its obviously past my ability to pin point.

I'm about over it and about to give up.
left field idea from a mate.

What if there is crud inside the full tank, namely rust. Over the 10 mins, it is clogging the outside of the pump filter sock, to the point its getting starved of fuel. When you turn it off and restart it, the crud falls away, and the process starts again?
The only three things I can think of the two O2 sensors having in common are the ground, the heater fuse, and the PCM. Ohm check your PCM ground to battery negative. The PCM ground is kind of feeble and the firewall to block ground is a known trouble spot, along with the dipstick ground connection.
Look closely at the heater fuse, fuses have been known to crack and make sometimes connections, depending on how hot they get.
Unlikely, but maybe due to a meltdown you are getting heater voltage into the O2 sensor to PCM circuit.
A quick test for ground health is to pick a spot and check for voltage between the ground wire and the battery negative. If a ground connection has enough resistance the current can back up in the ground wire. The term is standing voltage.
Add a 6 or 8 gauge wire from your battery negative to the inside of your fender, if there isn't already one there. A little insurance.

You have a real head-scratcher going on here.

The O2 sensors you put in, did they come with the correct connector or did you splice the O2 onto the existing connector?
Your on the money.
My mate suggested the exact same thing tonight.
So I got stuck into it, dropped the tank. Poured all the fuel out, it was filthy.
Acid washed it. Rinsed it out over and over and over.
Filled back up with fuel.
Took for test drive...


If I had a fuel pressure gauge, I guess I would of narrowed it down earlier. But the only ones locally only went to 10psi ?!?!

I will be definately investing in one.

Thanks for all the replies blokes.

At least I learnt heaps about the jeep, its components. Tested about 87 spare parts, sensors, leads, rotors, etc. at least I know they are all good. Jeep has had a good tune up as well.

Something so simple. I dont know why I didnt think of it earlier. What a doosh.

Thanks again all, and merry xmas from down under.
Congrats on getting it figured out.

I hope you and your son have a great holiday with many memories to look back on.
I haven't read the whole thread but being old school, rather than throw parts at it, I would do some basic tests.

What do the plugs look like...white/brown, buildup?
Wet and dry compression test
Vacuum test
Timing light test

If it is a weird problem, I always do all these tests. they can all be done in about a half hour.

My guess is probably a leaky intake manifold seal to the block. If you just gave me the symptoms, I would say IAC/cleaning, clogged CAT, cracked exhaust header or bad fuel pump.

Backfiring normally signifies timing but can be from running too lean and causing overheating of carbon and such in the cylinders or sucking air in the exhaust manifold and causing ignition in the tail pipe. When you say backfire, does it come out the throttle body or the tailpipe.